Solid electrolytic capacitor and method for producing the same

ABSTRACT

The present invention relates to a solid electrolytic capacitor having a masking structure in which the insulation between the anode part and the cathode part can be ensured without fail, to its production method, to a method for coating a masking agent on a solid electrolytic capacitor substrate, and to apparatus therefore. According to the present invention, the masking material covers the dielectric film on the metal material having valve action and sufficiently infiltrates into the core metal made of a metal having valve action while the solid electrolyte is masked by the masking material without fail, so that a solid electrolytic capacitor can be produced that has a reduced leakage current and a reduced stress generated at the reflow treatment or the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 10/876,622filed Jun. 28, 2004 now U.S. Pat. No. 7,046,504, which is a Divisionalof application Ser. No. 09/576,957 filed May 24, 2000, now U.S. Pat. No.6,890,363, which claims benefit of U.S. Provisional Application Nos.60/135,843 and 60/135,844 filed on May 24, 1999; the disclosures ofwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a solid electrolytic capacitor and toits production method. More specifically, the present invention relatesto a solid electrolytic capacitor having a masking structure that informing a solid electrolyte layer on a valve-acting metal substratehaving thereon a dielectric film, in which a portion of metal substratewhere the solid electrolyte layer is not provided (anode part) can beinsulated without fail from the solid electrolyte layer or anelectrically conducting layer formed on the solid electrolyte layerusing an electrically conducting paste or the like (cathode part) and toits production method. Also, the present invention relates to a methodand apparatus for coating a masking agent on the substrate of a solidelectrolytic capacitor.

BACKGROUND ART

A solid electrolytic capacitor using an electrically conducting polymerhas a basic structure such that an oxide dielectric film is formed onthe surface of a valve-acting metal such as aluminum, tantalum ortitanium, previously subjected to etching treatment, an electricallyconducting polymer which works out to solid electrolyte is formed on theoxide dielectric film, an anode lead is connected to the anode terminal(the metal surface area where the solid electrolyte is not formed) and acathode lead is connected to the electrically conducting layercontaining the electrically conducting polymer. The solid electrolyticcapacitor is manufactured by finally sealing the device as a whole withinsulating resins such as epoxy resins.

Such solid electrolytes using an electrically conducting polymer for thesolid electrolyte can be reduced in the equivalent series resistance andthe leakage current as compared with solid electrolytic capacitors usingmanganese dioxide or the like for the solid electrolyte. This isadvantageous in manufacturing a capacitor capable of coping with thetendency of electronic equipment toward higher performance and smallersize. Accordingly, a large number of production methods have beenproposed therefor.

In order to produce a high-performance solid electrolytic capacitorusing an electrically conducting polymer, it is indispensable to secureelectrical insulation of the anode part which works out to an anodeterminal, from the cathode part comprising an electrically conductinglayer containing an electrically conducting polymer.

As the masking means for insulating the anode part from the cathode partof a solid electrolytic capacitor, for example, a method of coating,printing or potting epoxy resins, phenol resins or the like on anunformed area and curing the resin to prevent passing of electricity(see, JP-A-3-95910 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”)), a method of electrodepositinga solution containing a polyamic salt on at least a part of thevalve-acting metal in the area where the solid electrolyte is notformed, thereby forming a polyamic acid film, and dehydration-curing thefilm by heating to form a polyimide film (see, JP-A-5-47611), a methodof forming a tape or resin coating film part made of polypropylene,polyester, silicon or fluorine-based resin so as to prevent the solidelectrolyte from climbing up (see, JP-A-5-166681), and a method offorming an insulating resin layer on the surface of a metal substrate inthe boundary part between the area which works out to an anode terminaland the area where the capacitor is formed, and removing the insulatingresin layer in the area other than the capacitor part to expose themetal substrate (see, JP-A-9-36003).

The method of using phenol resins or epoxy resins as the maskingmaterial (JP-A-3-95910) is disadvantageous in that the capacitor isgreatly damaged when pressed by external force, because the elasticmodulus of resin is high and the stress against strains is high.

The method of forming a polyimide film by electrodeposition(JP-A-5-47611) may successfully form a film even inside the pore partsas compared with ordinary coating methods, however, the production costincreases because of necessity of the electrodeposition step andmoreover, a dehydration step at a high temperature is necessary so as toform the polyimide film.

The method of forming an insulating resin-made tape or resin coatingfilm part so as to prevent the solid electrolyte from climbing up at themanufacturing (JP-A-5-166681) has difficulty in firmly fixing the tape(film) at edge parts of the substrate and bears a risk of polymer solidelectrolyte as the solid electrolyte invading the anode side.

The method of forming an insulating resin layer-and-then removing theinsulating resin layer in the area other than the capacitor part toexpose the metal substrate (JP-A-9-36003) includes a substantiallyuseless step of once forming an insulating resin layer and then removingit.

As described above, the conventional masking means are insufficient andactually it has not been clear yet what is like the form (structure) ofa masking that can insulate without fail the anode part from the cathodepart of a solid electrolytic capacitor.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a solidelectrolytic capacitor having a masking structure that in forming asolid electrolyte layer on a valve-acting metal substrate having thereona dielectric film, in which a portion of metal substrate where the solidelectrolyte layer is not provided (anode part) can be insulated withoutfail from the solid electrolyte layer or an electrically conductinglayer formed on the solid electrolyte layer using an electricallyconducting paste or the like (cathode part) and an its productionmethod.

Another object of the present invention is to provide a method forcoating a masking agent that can efficiently perform masking capable ofinsulating without fail the anode part from the cathode part of a solidelectrolytic capacitor and an apparatus therefor.

SUMMARY OF THE INVENTION

The present inventors have made extensive investigations on thefollowing matters: (1) to use as a material for forming a maskingmaterial (hereinafter referred to as a masking material coatingsolution), for example, a solution containing a heat resistant resin ora precursor thereof, or a solution containing a low molecular weightpolyimide having good insulating property and exhibiting high heatresistance after the curing or a precursor thereof; (2) to applyelectrochemical forming to the cut out parts after the cutting of ametal material because in the case of conventional metal formed materialfoils, the cut out parts generated on the cutting into a desireddimension remain unformed and give rise to an increase in the leakagecurrent; and (3) to use as the solid electrolyte an electricallyconducting polymer containing as a repeating unit a divalent group ofany one of pyrrole, thiophene, aniline and furan or any substitutedderivative thereof. As a result, it has been found that a maskingstructure can be formed in which the masking material is attached on thedielectric film sufficiently and infiltrated into the dielectric film tothe core metal. The present invention has been accomplished based onthis finding.

Also, the present inventors have made extensive investigations on thematter that upon electrochemical formation treatment, theelectrochemical formation liquid infiltrates into the substrate toincrease the leakage current at the time of electrochemical formation sothat short circuit to the electrode occurs. As a result, it has beenconfirmed that dividing the masking step into two stages and applying atemporary masking (first masking layer), performing electrochemicalformation treatment based on the position of the temporary masking andapplying main masking (second masking layer) on different portions ofthe substrate so that necessary portions (portions except for the anodepart of the solid electrolytic capacitor) can be formed without fail andeasily, causing no infiltration of the electrochemical formation liquidacross the temporary masking layer.

Further, the present inventors conducted studies in an attempt todiscover a method for applying a heat resistant resin having goodinsulating property and heat resistance, such as polyimide, as a maskingmaterial to an etched valve-acting metal substrate and having a metaloxide layer formed on the surface, at a desired position (around anentire circumference).

The present inventors examined the following methods:

(1) A masking agent in the form of a fine string is hung down directlyonto the surface of a substrate (formed aluminum foil) by means of, forexample, a dispenser;

(2) A masking agent is applied to the surface of formed aluminum foil bymeans of, for example, a brush or a slender bar such as a bamboo skewer;and

(3) A masking agent is applied to the surface of formed aluminum foilthrough screen printing.

Methods (1) and (2) can apply the masking agent within a short period oftime, but involve difficulty in maintaining stable work over a longperiod of time, because of partial solidification on, for example, abamboo skewer. Further, since the surface of a typical porous formedaluminum foil repels the masking agent, linear application of themasking agent is difficult, and thus the applied masking agent tends tobecome nonuniform. The screen printing method described in (3) canuniformly apply the masking agent to the foil surface, but involvesdifficulty in applying the masking agent to a predetermined thickness(about 10–30 μm/one surface) and in applying the masking agent reliablyto side surfaces of the formed foil.

As described above, any one of the above methods involve difficulty inapplying a masking agent uniformly and linearly around an entirecircumference of a substrate.

Next, in terms of efficient application of a masking agent to a numberof substrates (formed aluminum foils), the present inventors have foundpromising a method in which a plurality of substrates are connected to aguide plate in a cantilever fashion, and a masking agent is applied toeach of the substrates at a predetermined position and around an entirecircumference thereof. Thus, the present inventors trial-manufactured,for study, an apparatus including: a device for moving a metallic guideto which the substrates are fixedly attached; a rotating disk having acircumferential surface which serves as a coating surface; a bath whichcontains a masking agent and allows the rotating disk to be partiallyimmersed in the masking agent; and a scraper for scraping off residualmaterial from the rotating disk. The circumferential surface, to whichthe masking agent adheres, is brought into contact with the lowersurface of the formed aluminum foil connected to the metallic guideplate, thereby applying the masking agent.

However, the trial-manufactured apparatus involves the followingproblems. Since a solution which contains a masking material iscontained in an open system (exposed to the air) during coating, themasking material is solidified in the vicinity of the scraper. Also, theviscosity of the masking agent contained in the bath varies, causinginstability in coating. Thus, the solution which contains the maskingmaterial must be replaced at short intervals.

The present inventors have been successful in applying a masking agentuniformly and linearly around an entire circumference of a substratethrough employment of the following measures.

(1) A plurality of formed foils (substrates) are fixedly attached, in acantilever fashion, to a base (metallic guide) which moves linearly.

(2) A rotating disk is disposed such that the top of a smoothcircumferential surface (coating surface) abuts, at a constant force,the back surface (lower surface) of the substrate, which is fixedlyattached to the metallic guide.

(3) A solution which contains a masking material is stored in a closedcontainer, and the masking agent is fed to the coating surface of therotating disk through a closed system; specifically, through, forexample, a resin tube or needle by use of a quantity coating-fluidfeeder, such as a continuous quantity dispenser of little pulsation.

(4) The rotating disk, whose circumferential surface serves as thecoating surface and is uniformly coated with the solution which containsthe masking material, is pressed against the formed foil, therebyapplying the masking material to the lower surface and side surfaces ofthe formed foil substrate through adjustment of the traveling speed ofthe metallic guide plate and the rotational speed of the rotating disk.

(5) There is provided means for cleaning off the remaining maskingmaterial from a portion of the coating surface located downstream of theposition where the rotating disk comes into contact with the formed foilsubstrate and upstream of the position where the rotating disk is coatedwith fresh coating solution.

That is, the present invention provides a method for producing a solidelectrolytic capacitor, a solid electrolytic capacitor produced by themethod, a method for coating a masking material solution and anapparatus therefor as described below.

[1] A method for producing a solid electrolytic capacitor comprising ametal material having thereon a dielectric film and a solid electrolyteformed on a desired position of the dielectric film, the metal materialhaving valve action, wherein the method comprises the step of coating amasking material solution that infiltrates into the dielectric film andforms a masking layer on the infiltrated portion.[2] A method for producing a solid electrolytic capacitor comprising ametal material having thereon a dielectric film and a solid electrolyteformed on a desired position of the dielectric film, the metal materialhaving valve action, wherein the method comprises the step of coating amasking material solution that infiltrates into the dielectric film andforms a masking layer on the infiltrated portion, wherein a maskingresin that has infiltrated into the dielectric film and solidifiedduring the coating step prevents infiltration of a solid electrolyteformed in a subsequent step.[3] The method for producing a solid electrolytic capacitor as describedin [2] above, wherein the concentration of the solid electrolyte in thedielectric film where the masking resin has infiltrated in the step ofcoating a masking material solution is not higher than a detection limitvalue attained by use of an electron probe microanalyser.[4] The method for producing a solid electrolytic capacitor as describedin any one of [1]to [3] above, wherein a plurality of solid electrolyticcapacitor substrates are fixedly attached to a metallic guide in acantilever fashion, and a rotating disk is brought into contact with thesubstrates at a desired position at a predetermined pressing force whilethe metallic guide is moved, thereby coating a masking materialsolution, which is fed from masking-material-solution supply means tothe coating surface of the rotating disk, on opposite surfaces andopposite side surfaces of the solid electrolytic capacitor substrate ata desired position to form the masking layer.[5] The method for producing a solid electrolytic capacitor as describedin [4] above, wherein the relative position between the metallic guideand the rotating disk is inverted to thereby apply the masking materialsolution to opposite surfaces and opposite side surfaces of thesubstrate fixedly attached to the metallic guide.[6] A method for producing a solid electrolytic capacitor comprising ametal material having thereon a dielectric film and a solid electrolyteformed on a desired position of the dielectric film, said metal materialbeing cut into a predetermined shape and having valve action, asdescribed in [1] above wherein the method comprises the step of coatinga masking material solution on said metal material to form a firstmasking layer and the step of coating a masking material solution onsaid metal material to form a second masking layer, wherein at least thestep of forming a second masking layer causes the infiltration of themasking material solution into the dielectric film and the formation ofthe masking layer on the infiltrated portion.[7] A method for producing a solid electrolytic capacitor comprising ametal material having thereon a dielectric film and a solid electrolyteformed on a desired position of the dielectric film, said metal materialbeing cut into a predetermined shape and having valve action, whereinthe method comprises:

a step of linearly coating a masking material solution around the entirecircumference in the region undertaking the boundary in the applicationof electrochemical forming onto said metal material, and heating thesolution to form a first masking layer;

a step of subjecting an area where a solid electrolyte is formed laterto electrochemical forming, the area being defined by the first maskinglayer on said metal material;

a step of further linearly coating a masking material solution aroundthe entire circumference in the region at a predetermined distance fromsaid first masking layer on said electrochemically formed metalmaterial, and heating the solution to form a second masking layer;

a step of forming a solid electrolyte in the area exclusive of the spacebetween said first masking layer and said second masking layer out ofthe area subjected to said electrochemical forming; and

a step of cutting said metal material in the space between said firstmasking layer and said second masking layer.

[8] The method for producing a solid electrolytic capacitor as describedin any one of [1] to [7] above, wherein a solution of a heat resistantresin or a precursor thereof is used as the masking material solution.

[9] The method for producing a solid electrolytic capacitor as describedin [8] above, wherein the solution of a heat resistant resin or aprecursor thereof is a low molecular weight polyimide solution orpolyamic acid solution capable of being solidified by heating.[10] The method for producing a solid electrolytic capacitor asdescribed in [8] or [9] above, wherein the masking material solutionfurther contains silicone oil, silane coupling agent orpolyimidesiloxane.[11] The method for producing a solid electrolytic capacitor asdescribed in any one of [1] to [7] above, wherein the metal foilmaterial having valve action is a metal material selected from the groupconsisting of aluminum, tantalum, niobium, titanium, zirconium, and analloy thereof.[12] The method for producing a solid electrolytic capacitor asdescribed in any one of [1] to [7] above, wherein the solid electrolyteis a polymer solid electrolyte containing as a repeating unit at leastone of a divalent group of any one of pyrrole, thiophene, aniline andfuran or any substituted derivative thereof.[13] The method for producing a solid electrolytic capacitor asdescribed in [12] above, wherein the solid electrolyte contains apolymer of 3,4-ethylenedioxythiophene.[14] The method for producing a solid electrolytic capacitor asdescribed in [12] or [13], wherein the polymer solid electrolyte furthercontains a dopant of an arylsulfonic salt.[15] A solid electrolytic capacitor comprising a metal material havingthereon a dielectric film and a solid electrolyte formed on a desiredposition of the dielectric film, the metal material having valve action,wherein said solid electrolytic capacitor comprises a structure in whicha masking material solution has infiltrated into said dielectric filmand forms a masking layer on the infiltrated portion of the dielectricfilm, so that the solid electrolyte is prevented from infiltrating intothe dielectric film where the masking material solution has infiltratedand masked by the masking layer formed on the infiltrated portion.[16] The solid electrolytic capacitor as described in [15] above,wherein the masking layer is formed using a masking material solution ofa heat resistant resin or precursor thereof.[17] The solid electrolytic capacitor as described in [15] above,wherein the concentration of the solid electrolyte in the dielectricfilm where the masking material solution has infiltrated is not higherthan a detection limit value attained by use of an electron probemicroanalyzer.[18] A method for coating a masking agent, comprising the steps offixedly attaching a plurality of solid electrolytic capacitor substratesto a metallic guide in a cantilever fashion, and bringing a rotatingdisk into contact with the substrates at a desired position at apredetermined pressing force while the metallic guide is moved, therebycoating a masking material solution, which is fed from masking materialsolution supply means to the coating surface of the rotating disk, toopposite surfaces and opposite side surfaces of the solid electrolyticcapacitor substrate at a desired position.[19] The method for coating a masking agent as described in [18] above,wherein the relative position between the metallic guide and therotating disk is inverted to thereby apply the masking material solutionto opposite surfaces and opposite side surfaces of the substrate fixedlyattached to the metallic guide.[20] An apparatus for coating a masking agent to opposite surfaces andopposite side surfaces of the solid electrolytic capacitor substrate(12) at a desired position, comprising a metallic guide (11) to which aplurality of solid electrolytic capacitor substrates (12) are fixedlyattached in a cantilever fashion; means for moving said metallic guide;a rotating disk (13) which comes into contact with the substrates (12)at a desired position at a predetermined pressing force; means (14) forfeeding to the coating surface of said rotating disk (13) a solutionwhich contains the masking material; and a scraper (15) for cleaning thecoating surface of said rotating-disk (13).[21] The apparatus for coating a masking agent as described in [20]above, wherein the relative position between the metallic guide and therotating disk is inverted to thereby apply the masking material solutionto opposite surfaces and opposite side surfaces of the substrate fixedlyattached to the metallic guide.[22] The apparatus for coating a masking agent as described in [20]above, wherein two rotating disks are employed, and either one of thetwo rotating disks is dedicated to coating the masking material solutionto reversal surfaces of the substrates fixedly attached to the invertedmetallic guide.[23] The apparatus for coating a masking agent as described in [20]above, wherein two rotating disks are disposed on opposite sides withrespect to the substrates fixedly attached to the metallic guide,thereby coating the masking material solution concurrently to oppositesurfaces and opposite side surfaces of the substrate.[24] The apparatus for coating a masking agent as described in any oneof [20] to [23] above, wherein the substrate is formed of a valve-actingmetal, and the coating surface of the rotating disk comes into contactwith the substrates at a pressing force which does not exceed theelastic limit of the substrate.[25] The apparatus for coating a masking agent as described in any oneof [20] to [23] above, wherein the rotating disk is formed of a steelmaterial or ceramic material.[26] The apparatus for coating a masking agent as described in any oneof [20] to [25], wherein the scraper is in the form of a blade whichmakes line contact with the coating surface of the rotating disk andwhich is formed of a resin or a steel softer than the material of therotating disk.[27] The apparatus for coating a masking agent as described in any oneof [20] to [26], wherein a wiping material (16) comprising resin fibersoaked with an organic solvent and/or water is disposed in front of thescraper.[28] The apparatus for coating a masking agent as described in any oneof [20] to [27], wherein means (14) for feeding the masking agentcomprises a continuous quantity dispenser and a tubular member.

FIGS. 1A to 1D show the outline of the production steps of a solidelectrolytic capacitor according to the present invention and FIG. 2 isa schematic diagram illustrating the structure of the masking layer ofthe resulting solid electrolytic capacitor.

FIG. 1A is a plan view of a metal material (1) having thereon a porousoxide film, which is cut into a predetermined size and works out to asubstrate of a solid electrolytic capacitor. FIG. 1B is a plan viewshowing the state where the first masking layer (2) is applied. FIG. 1Cis a plan view showing the state where a electrochemically formed layer(3) is provided so as to form a porous oxide film without fail on thecut end parts produced accompanying the cutting. FIG. 1D is a plan viewshowing the state where a second masking layer (4) is applied.

FIG. 2 shows the structure of the masking portion along the crosssection A–A′ at FIG. 1D in an enlarged scale. As shown in FIG. 2, in thepresent invention, the masking layer (2) enters into the dielectric film(1 b) and also is formed on the infiltrated portion while the solidelectrolyte which infiltrates into the dielectric film (1 b) cannotinfiltrate into the dielectric film into which the masking material hasalready infiltrated and has a structure completely masked by the maskinglayer formed on the infiltrated layer.

In practical solid electrolytic capacitor device (9) as seen in FIG. 3,a lead wire (6) is connected to the anode terminal (5) assumed by thecutting plane, a lead wire (7) is connected to the cathode assumed bythe solid electrolyte layer (4) or an electrically conducting layer (notshown) formed thereon using an electrically conducting paste or thelike, and the whole is moulded with an insulating resin (8) such asepoxy resins, thereby completing the solid electrolytic capacitor.

When the masking process is practiced twice, the solid electrolyticcapacitor can be produced by the process shown in FIGS. 4A to 4F.

FIG. 4A is a plan view of a metal material (1) having thereon a porousoxide film, which is cut into a predetermined size and works out to asubstrate of a solid electrolytic capacitor. FIG. 4B is a plan viewshowing the state where the first masking layer (2 a) is applied. FIG.4C is a plan view showing the state where a electrochemically formedlayer (3) is provided so as to form a porous oxide film without fail onthe cut end parts produced accompanying the cutting. FIG. 4D is a planview showing the state where a second masking layer (2 b) is applied.FIG. 4E is a plan view showing the state where the solid electrolyte (4)is formed. FIG. 4F is a plan view of a solid electrolytic capacitordevice (9) obtained by cutting between the first masking layer (2 a) andthe second masking layer (2 b).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D each is a view showing the outline of the productionprocess of a solid electrolytic capacitor according to the presentinvention.

FIG. 2 is a schematic diagram showing the structure of a masking layerof a solid electrolytic capacitor obtained by the method of the presentinvention.

FIG. 3 is a cross section of an example of the solid electrolyticcapacitor device.

FIGS. 4A to 4F each is a view showing the outline of the productionprocess of a solid electrolytic capacitor when two-stage maskingtreatment is performed.

FIG. 5A is a plan view of an example of an apparatus for coating amasking material and FIG. 5B is a side view of the example of theapparatus.

FIG. 6 is an explanatory view showing application of the maskingmaterial to side surfaces of the substrate.

FIG. 7 is a cross section of the solid electrolytic device manufacturedin Examples.

FIG. 8 is an enlarged photograph showing a peripheral portion of themasking portion of a capacitor coated with the masking material.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail below.

(Valve-Acting Metal)

The substrate of the solid electrolytic capacitor is a valve-actingmetal having on the surface thereof an oxide dielectric film. Thevalve-acting metal is selected from aluminum, tantalum, niobium,titanium and zirconium or a valve-acting metal foil or bar of an alloysystem comprising these metals as a substrate or sintered body mainlycomprising these metals. The metal has a dielectric film on the surfacethereof due to oxidation by oxygen in air. However, in order to ensurethe formation of dielectric oxide film, the metal is preferablysubjected to etching or the like in advance by a known method to roughenthe surface and then electrochemically formed in a usual manner. Thevalve-acting metal is preferably an aluminum foil having thereon analuminum oxide layer.

The valve-acting metal after the surface roughening treatment ispreferably cut into a size agreeing with the shape of the solidelectrolytic capacitor, before use.

The thickness of the valve-acting metal foil varies depending on the usepurpose, however, it is generally from about 40 to about 150 μm. Thesize and the shape of the valve-acting metal foil also Vary depending onthe use, however, the metal foil as a plate-form device unit preferablyhas a rectangular shape having a width of from about 1 to about 50 mmand a length of about 1 to about 50 mm, more preferably having a widthof about 2 to about 20 mm and a length of about 2 to about 20 mm, andmost preferably having a width of about 2 to about 5 mm and a length ofabout 2 to about 6 mm.

(Electrochemical Forming)

The valve-acting metal cut into a predetermined shape iselectrochemically formed by various methods. By previously performingelectrochemical forming, increase of the leakage current can beprevented even if defects are generated in the masking layer.

The conditions for electrochemical forming are not particularly limited.For example, the electrochemical forming is performed using anelectrolytic solution containing at least one of oxalic acid, adipicacid, boric acid, phosphoric acid and the like under the conditions suchthat the concentration of electrolytic solution is from 0.05 to 20 wt %,the temperature is from 0 to 90° C., the current density is from 0.1 to200 mA/cm², the voltage is a numerical value selected according to theelectrochemical forming voltage of a film already present on the formedfoil to be treated, and the electrochemical forming time is 60 minutesor less. More preferably, the conditions are selected in the range suchthat the concentration of electrolytic solution is from 0.1 to 15 wt %,the temperature is from 20 to 70° C., the current density is from 1 to100 mA/cm² and the electrochemical forming time is 30 minutes or less.

The above-described conditions for electrochemical forming are suitablyused in industry, however, as long as the oxide dielectric film alreadyformed on the surface of the valve-acting metal material is not rupturedor deteriorated, various conditions such as kind of electrolyticsolution, concentration of electrolytic solution, temperature, currentdensity, electrochemical forming time and the like, may be freelyselected.

If desired or needed, the metal material may be subjected before orafter the electrochemical forming to a treatment such as dipping inphosphoric acid for improving the water resistance, heating or dippingin boiling water for strengthening the film.

(Masking Material)

The masking layer is provided so as to prevent the electrochemicalforming solution from running out to the portion which works out toanode of a solid electrolytic capacitor and ensure insulation from asolid electrolyte (cathode portion) to be formed in the subsequent step.

Accordingly, for the masking material, a composition comprising commonlyused heat-resistant resins, preferably heat-resistant resins orprecursors thereof soluble or swellable in a solvent, and furthercontaining inorganic fine particle and cellulose-based resins (seeJP-A-11-80596) may be used, but the material is not limited. Specificexamples thereof include polyphenylsulfones (PPS), polyethersulfones(PES), cyanic ester resins, fluororesins (e.g., tetrafluoroethylene,tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers), lowmolecular weight polyimides and derivatives thereof. Among these, lowmolecular weight polyimides, polyethersulfones, fluororesins andprecursors thereof are preferred.

When the masking step is performed twice, the first masking layer isprovided so as to prevent the electrochemical forming solution fromrunning out to the anode portion of the solid electrolytic capacitor.The material for the first masking material is not particularly limitedbut the above-described commonly used heat resistant resins can be used.For the second masking, the same material for the first masking materialmay be used. In particular, polyimides are preferred because of itssufficiently high adhesive strength to the valve-acting metal, goodfilling property, capability of withstanding a heat treatment up toabout 450° C. and excellent insulating property.

Polyimides are conventionally used as a solution prepared by dissolvinga polyamic acid as a precursor in a solvent and after the coating, thesolution is imidized by a heat treatment at a high temperature. Thus, aheat treatment at from 250 to 350° C. is necessary and this causes aproblem such as rupture of the dielectric layer on the surface of anodefoil due to the heat.

Polyimides used in the present invention are satisfactorily curable by aheat treatment at a low temperature of 200° C. or less, preferably from100 to 200° C., and reduced in the external shocks such as rupture orbreakage of the dielectric layer on the surface of anode foil.

The polyimides are a compound containing an imide structure in the mainchain. Examples of the polyimides which can be preferably used in thepresent invention include the compounds represented by the followingformulae (1) to (4) each having a flexible structure whereintramolecular rotation readily takes place in the diamine componentskeleton, and polyimides represented by the following formula (5)obtained by the polycondensation reaction of3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride with aromaticdiamines. The polyimides preferably have an average molecular weight offrom about 1,000 to about 1,000,000, more preferably from about 2,000 toabout 200,000.

These compounds each can be dissolved or dispersed in organic solvents,therefore, a solution or dispersion having a solid concentration (inturn viscosity) suitable for the coating operation can be easilyprepared. The concentration is preferably from about 10 to about 60 wt%, more preferably from about 15 to about 40 wt %. The viscosity ispreferably from about 50 to about 30,000 cp, more preferably from about500 to about 15,000 cp. If the concentration or viscosity is less thanthis range, the masking line may be blurred, whereas if theconcentration or viscosity exceeds the range, cobwebbing or the likeoccurs and an unstable line width results.

After the coating of a masking material solution, the formed maskinglayer may be subjected to treatments such as drying, heating,irradiation with light and the like, if desired.

Specific examples of the polyimide solution which can be preferably usedinclude a solution obtained by dissolving low molecular polyimidescurable by a heat treatment after the coating, in a solvent reduced inthe hygroscopicity such as 2-methoxyethyl ether and triethylene glycoldimethyl ether (the solution is commercially available, for example,under the trademark of “UPICOAT™ FS-100L” from Ube Industries, Ltd.),and a solution obtained by dissolving polyimide resins represented byformula (5) in NMP (N-methyl-2-pyrrolidone) or DMAc (dimethylacetamide)(the solution is commercially available, for example, under thetrademark of “RIKACOAT™” from Shin Nippon Rika K. K.).

In the former case, the solution coated is thermally modified into apolymer and cured by a heat treatment at from 160 to 180° C., as aresult, a flexible film having high heat resistance and good insulatingproperty is formed. The polyimide film obtained maintains rubber-likeproperties such that the tensile strength is 2.0 kg/mm², the elongationof cured film is 65% and the initial elastic modulus is 40.6 kg/mm², andat the same time exhibits high heat resistance such that the thermaldecomposition temperature is 461° C. Furthermore, the volume resistivityis as high as 10¹⁶ Ω·cm even under humidification and the dielectricconstant is as low as 3.2, thus the polyimide film holds excellentelectrical properties as the insulating film.

In the latter case, a film excellent in the heat resistance, mechanicalproperties, electrical properties and resistance against chemicals canbe formed merely by removing the solvent at a temperature of 200° C. orless. The film obtained has a tensile strength of about 11.8 kg/mm², acured film elongation of 14.2%, an initial elastic modulus of 274 kg/mm²or more and a heat resistance such that the temperature for weightreduction of 5% is 515° C. The volume resistivity is 10¹⁶ Ω·cm and thedielectric constant is 3.1 (25° C.) or 2.8 (200° C.), thus, the filmholds excellent electrical properties.

In the present invention, the masking material solution may contain adefoaming agent (e.g., lower alcohol type, mineral oil type, siliconeresins type, oleic acid, polypropylene glycol), a thixotropy-impartingagent (e.g., silica fine powder, mica, talc, calcium carbonate) or asilicon agent for resin modification (e.g., silane coupling agent,silicone oil, silicon-based surfactant, silicone-based syntheticlubricant). For example, by adding a silicone oil (e.g., polysiloxane)and a silane coupling agent, improvements may be expected in thedefoaming property (to prevent bubbling at curing), releasability (toprevent adhering of electrically conducting polymer), lubricity(permeability inside the pore part), electrical insulating property (toprevent leakage current), water repellency (to prevent climbing up ofthe solution at the polymerization of electrically conducting polymer),damping and vibration proofing property (to oppose pressure at thestacking of capacitor device), or heat resistance and weatherability ofresin (introduction of crosslinking mechanism).

In the present invention, the above-described effects by virtue ofaddition of silicone oil (polysiloxane) may also be similarly obtainedby using a composition comprising soluble polyimidesiloxane and epoxyresin (see, JP-A-8-253677 (U.S. Pat. No. 5,643,986)).

(Coating Method of Masking Material)

An apparatus for coating a masking agent to a solid electrolyticcapacitor substrate (hereinafter referred to simply as “substrate”)according to the present invention will next be described with referenceto a plan view (FIG. 5A) and a side view (FIG. 5B) schematically showingan embodiment of the present invention.

The apparatus of FIG. 5A is adapted to carry out, during one rotation ofa disk (13), one cycle consisting of the steps of: feeding to thecoating surface of the disk a solution which contains a maskingmaterial; coating the masking agent to a substrate; and cleaning off theremaining masking agent from the coating surface of the disk.

In FIG. 5 reference numeral 11 denotes a metallic guide to which aplurality of substrates (12 a, 12 b, 12 c, . . . ) are fixedly attachedin a cantilever fashion.

The substrates can be fixedly attached to the metallic guide (11)through electrical or mechanical bonding. Examples of a bonding methodinclude soldering, bonding by use of an electrically conducting paste,ultrasonic welding, spot welding, and electron beam welding.

The metallic guide (11) moves linearly around over the rotating disk(13) in the direction of the arrow. Customarily used means, such as amotor, belt, or cylinder, may be used as means (not shown) for movingthe metallic guide (11). Through adjustment of the relationship betweenthe traveling speed (V1) of the metallic guide and the rotational speed(V2) of the rotating disk, the side surfaces of the substrate (12) canbe coated with the masking agent. Specifically, as shown in FIG. 6A,when the traveling speed (V1) of the metallic guide is greater than therotational speed (V2), the side surface of the substrate located forwardwith respect to the traveling direction can be coated with the maskingagent. When the traveling speed (V1) of the metallic guide is smallerthan the rotational speed (V2), the side surface of the substratelocated backward with respect to the traveling direction can be coatedwith the masking agent (FIG. 6B).

The apparatus of the present invention also includes a mechanism forinverting the relative position between the metallic guide and therotating disk, thereby coating the masking agent to opposite surfacesand opposite side surfaces of substrates fixedly attached to themetallic guide.

An example of such a mechanism is an inverting mechanism for use withthe metallic guide. Inversion can be carried out by use of a motor orcylinder, or by rendering a holder of the metallic guide rotatable aboutthe longitudinal direction of the metallic guide and turning the holderby half-turn.

In order to apply the masking agent to the reversal surfaces of thesubstrates fixedly attached to the inverted metallic guide, therelationship between the coating surface of the rotating disk and thecoating position of the substrates must be adjusted. This adjustment canbe performed by moving the metallic guide, moving the rotating disk, ormoving both metallic guide and rotating disk by use of customarily usedmeans.

Alternatively, two rotating disks may be employed. Either one of the tworotating disks is dedicated to coating of the masking agent to uncoatedsurfaces of the substrates fixedly attached to the inverted metallicguide.

Further, two rotating disks may be disposed on opposite sides withrespect to the substrates fixedly attached to the metallic guide,thereby coating the masking agent concurrently to opposite surfaces andopposite side surfaces of the substrate.

The rotating disk (13) is a disk-shaped roll that has a smooth coatingsurface, which comes into contact with the substrate at a desiredposition at a predetermined pressing force.

The rotating disk (13) is formed of a hard material resistance to asolution which contains the masking material; specifically, a metal(stainless steel, for example) or a ceramic material. The size of therotating disk (13) may be such that the masking agent does notdegenerate during one rotation of the rotating disk (13) (during onecycle of coating of the masking agent). The size of the rotating disk(13) is usually about 2 mm to about 500 mm in diameter, but is notparticularly limited in the present invention. The width of the coatingsurface of the rotating disk is selected so as to apply the maskingagent by a desired width, and is preferably approximately 0.2 mm to 3.0mm.

Around the rotating disk (13), there are disposed means (14) for feedingto the coating surface of the rotating disk a solution which containsthe masking agent upstream of the position of contact between therotating disk (13) (upstream with respect to the direction of rotation),and a scraper (15) and a wiping material (16) for cleaning the coatingsurface of the rotating disk downstream of the position of contactbetween the substrate (12).

The present embodiment employs, as the means (14) for feeding thesolution which contains the masking material, a quantity coating-fluidfeeder equipped with a closed continuous quantity dispenser of littlepulsation. The quantity coating-fluid feeder feeds the solution whichcontains the masking material, to the coating surface of the rotatingdisk continuously at a predetermined flow rate through a resin tuberesistant to the solution and by means of a discharge needle.

In order to stably feed to the coating surface of the disk (13) thesolution which contains the masking material, there is provided a fineregulating mechanism for stabilizing the distance between the tip of theneedle and the disk surface. An example of such a mechanism is amicrometer head (screw mechanism) adapted to adjust a vertical positionfinely.

The coating surface of the rotating disk (13) fed with the solutionwhich contains the masking material comes into contact with thesubstrate at a constant pressing force at the position of contact withthe substrate. The pressing force is determined so as not to exceed theelastic limit of the substrate and preferably such that the deflectionof the rotating disk as measured at the smooth top (coating surface)falls within a range of from about 0.03 mm to about 0.3 mm. A specificpressing force depends on the type and thickness of the substrate, butmay be set to, for example, about 0.002 g to about 0.02 g per substrate(3 mm (width)×0.1 mm (thickness)).

Next, the coating surface of the disk is cleaned. For example, themechanical scraper (15) and the wiping material (16) are used ascleaning means.

The scraper (15) is the form of a blade which is formed of stainlesssteel or ceramic material as in the case of the disk or formed of amaterial (for example, resin or steel) softer than the material of thedisk. The scraper (15) is disposed such that at least a tip comes intoclose contact with the coating surface of the disk so as to scrape off aremained masking agent from the coating surface of the disk. The wipingmaterial (16) is disposed downstream of the scraper (15) (downstreamwith respect to the direction of rotation) and includes resin fibersoaked with an organic solvent and/or water (for example, a solventidentical to that used in the masking material solution). The wipingmaterial (16) is adapted to wipe off adhering substances from thecoating surface of the disk, thereby preparing for the next coatingcycle.

Since the apparatus of the present invention employs a nonpulsatingdrive system for feeding the solution which contains the maskingmaterial, from the closed container to the coating surface of therotating disk, the solution can be fed stably.

(Solid Electrolyte)

In the present invention, the solid electrolyte is preferably anelectrically conducting polymer containing as a repeating unit adivalent group of any one of pyrrole, thiophene, furan and anilinestructures, or any substituted derivative thereof. However, thoseconventionally known as the material for the solid electrolyte may beused without any particular limitation.

For example, a method of coating a 3,4-ethylenedioxy-thiophene monomerand an oxidizing agent each preferably in the form of a solutionseparately one after another or simultaneously on the oxide film ofmetal foil (see, JP-A-2-15611 (U.S. Pat. No. 4,910,645) andJP-A-10-32145 (EP 820076 (A2))) may be used.

In general, arylsulfonate-type dopants are used in the electricallyconducting polymer. Examples of the salt which can be used include saltsof benzenesulfonic acid, toluenesulfonic acid, naphthalenesulfonic acid,anthracene-sulfonic acid and anthraquinonesulfonic acid.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below by referring to Examples,however, the present invention should not be construed as being limitedto the following Examples.

EXAMPLE 1

Masking Step

A 100 μm-thick formed aluminum foil cut (slit) into a width of 3 mm wascut into strips each having a length of 13 mm. One short side part of afoil strip was fixed to a metal-made guide by welding. A polyimide resinsolution (UPICOAT™ FS-100L, produced by Ube Industries, Ltd.) adjustedto a viscosity of 800 cp was supplied to a disk-like coating apparatushaving a 0.4 mm-width coating surface and while press-contacting thecoating surface of the coating apparatus onto the aluminum formed foil,a 0.8 mm-width line was drawn by the solution on the portion 7 mm insidefrom the unfixed end. The solution was then dried at about 180° C. toform a masking layer (polyimide film).

Electrochemical Forming Step

The aluminum foil fixed to a metal-made guide was disengaged from thecoating apparatus. Thereafter, the area from the distal end to themasking line of the aluminum foil was dipped in an aqueous ammoniumadipate solution and a voltage of 13 V was applied to electrochemicallyform the unformed area on the cut end portion. Thus, a dielectric filmwas formed.

Solid Electrolyte Forming Step

In the electrochemically formed layer region, a solid electrolyte wasformed as follows.

More specifically, the area (3 mm×4 mm) opposite to the part of maskinglayer formed 4 mm inside from the distal end of aluminum foil was dippedin an isopropanol solution containing 20 wt % of3,4-ethylenedioxythiohene (Solution 1), pulled up and left standing at25° C. for 5 minutes. Thereafter, the aluminum foil area treated withthe monomer solution was dipped in an aqueous solution containing 30 wt% of aqueous ammonium sulfate solution (Solution 2) and then dried at60° C. for 10 minutes to allow oxidative polymerization to proceed. Theoperation from dipping in Solution 1 to oxidative polymerization bydipping in Solution 2 was repeated 25 times to form a solid electrolytelayer.

Fabrication and Test of Chip-Type Solid Electrolytic Capacitor Device

Three sheets of cut foil in the moiety having the masking layer weresuperposed by joining one on another with silver paste, an anode leadterminal was welded to the portion free of the electrically conductingpolymer, the whole was moulded with epoxy resin, and the device obtainedwas aged at 120° C. for 2 hours while applying a rated voltage. In thismanner, 30 units in total of chip-type solid electrolytic capacitorswere manufactured. FIG. 7 shows a cross section of the thus-manufacturedchip-type solid electrolytic capacitor.

These capacitor devices were subjected to a reflow soldering test bypassing each device through a temperature zone at 230° C. for 30seconds. The leakage current 1 minute after the application of a ratedvoltage was measured and an average leakage current of devices having ameasured value of 1 CV or less was determined. When the leakage currentwas 0.04 CV or more, the device was determined as defective. The resultsobtained are shown in Table 1.

Structural Analysis of the Masking Portion of Capacitor

In the solid electrolyte forming step of Example 1, aluminum foil wasused as the metal material and a sulfur-containing polymer layer (apolymer of 3,4-ethylenedioxy-thiphene) was used as a solid electrolytelayer. Thus produced capacitor material (specimen) was put in an epoxyresin (trade name: Quetol-812) and the resin was cured by heating at 30to 60° C. for 20 to 30 hours to fix the specimen. Then, it was cut alongA–A′ in FIG. 1D. FIG. 8 is a photograph (magnification: ×500) of theperipheral area of the masking portion.

Cutting was performed by means of a microtome along A–A′ and the cutsurface was observed for the two-dimensional distribution of a specificelement by a mapping method using an electron probe microanalyser(EPMA), which is an apparatus for analysing the composition of elementscontained in a minute volume (on the order of 1 μm³). Use of the aboveelectron probe microanalyser allows quantitative analysis of elementsfrom 1 up to several wt percents (%) per unit volume (μm³).

In the enlarged photograph, an aluminum core metal (1 a), dielectricfilm layer (1 b), a solid electrolyte (sulfur-containing polymer layer)(4), and a masking layer (2) are observed.

(1 a) and (1 b) contain aluminum element, (4) contains sulfur element,and (4) and (2) contain carbon element. Therefore, elemental analysis ofcarbon, sulfur, aluminum, etc. gave a clear distribution of maskingmaterial and polymer for each of the sites (1 a), (1 b), (4), and (2).FIG. 2 schematically illustrates how they distributed.

Observation of the distribution of detected sulfur elements (S) clearlyindicated isolation of the area where the electrolyte (4) distributed inthe dielectric layer (1 b), which demonstrated that the masking materialblocks the infiltration of the solid electrolyte. The term “blocking theinfiltration as used herein means that there exists no more than 5 wt %of the solid electrolyte material is present in the area where themasking agent has infiltrated.” This value can be obtained, for example,from a detection limit value for an element for identification of theelectron probe microanalyser and the content of the element foridentification in the solid electrolyte.

From the above results, the reason why formation of a capacitor deviceby the solid electrolytic capacitor of the present invention gives riseto capacitor characteristics improved in leakage current, capacitance,etc. is that the capacitor has a structure such that the masking agentinfiltrates into the dielectric film and forms a masking material on theinfiltrated portion of the dielectric film while the solid electrolytecannot infiltrate into the dielectric film where the masking agent hasinfiltrated and thus is completely blocked by the masking materialformed on the infiltrated portion of the dielectric film.

EXAMPLE 2

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 1 except for using a polyimide resin solution(RIKACOAT™, produced by Shin Nippon Rika K. K.) as the masking material.The measurement of leakage current and the reflow soldering test werealso performed in the same manner. The results obtained are shown inTable 1.

EXAMPLE 3

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 1 except that the oxidative polymerization in thestep of forming a solid electrolyte was performed by dipping thealuminum foil in an aqueous solution prepared by further adding sodium2-anthraquinonesulfonate (produced by Tokyo Chemical Industry Co.) toSolution 2 to have a concentration of 0.07 wt %. The measurement ofleakage current and the reflow soldering test were also performed in thesame manner. The results obtained are shown in Table 1.

EXAMPLE 4

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 1 except that the oxidative polymerization in thestep of forming a solid electrolyte was performed by dipping thealuminum foil in an aqueous solution prepared by further adding sodium2-naphthalenesulfonate (produced by Tokyo Chemical Industry Co.) toSolution 2 to have a concentration of 0.06 wt %. The measurement ofleakage current and the reflow soldering test were also performed in thesame manner. The results obtained are shown in Table 1.

EXAMPLE 5

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 3 except that N-methylpyrrole was used instead of3,4-ethylenedioxythiophene. The measurement of leakage current and thereflow soldering test were also performed in the same manner. Theresults obtained are shown in Table 1.

COMPARATIVE EXAMPLE 1

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 1 except that a tape comprising a heat resistantsubstrate and a heat resistant adhesive was bonded in the width of 1 mmto the front and back surfaces of the aluminum foil in place of formingthe masking layer. The measurement of leakage current and the reflowsoldering test were performed in the same manner. The results obtainedare shown in Table 1. Also, analysis of sulfur element using EPMA wasperformed in the same manner as in Example 1. The results indicated thatin the dielectric layer (1 b), the area where the solid electrolytelayer (4) distributed did not separate clearly but the solid electrolyteinfiltrated into the masking layer.

COMPARATIVE EXAMPLE 2

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 1 except that a 0.8 mm-width line was drawn on thefront and back surfaces of the foil by coating and curing phenol resinin place of forming a polymer insulating film. The measurement ofleakage current and the reflow soldering test were performed in the samemanner. The results obtained are shown in Table 1. Also, analysis ofEPMA was performed in the same manner as in Comparative Example 1. Theresults indicated that the solid electrolyte infiltrated into themasking layer.

TABLE 1 Heat Resistance Failure Average Ratio in Reflow Leakage CurrentSoldering Test Example 1 0.19 μA 0/30 Example 2 0.20 μA 0/30 Example 30.17 μA 0/30 Example 4 0.16 μA 0/30 Example 5 0.18 μA 0/30 ComparativeExample 1  2.0 μA 4/30 Comparative Example 2  2.2 μA 5/30

EXAMPLE 6

First Masking Step

A 100 μm-thick formed aluminum foil cut (slit) into a width of 3 mm wascut into strips each having a length of 13 mm. One short side part of afoil strip was fixed to a metal-made guide by welding. A polyimide resinsolution (RIKACOAT™, trademark, produced by Shin Nippon Rika K. K.)adjusted to a viscosity of 800 cp was supplied to a disk-like coatingapparatus having a 0.4 mm-width coating surface and whilepress-contacting the coating surface of the coating apparatus onto thealuminum formed foil, a 0.8 mm-width line was drawn by the solution onthe portion 7 mm inside from the unfixed end. The solution was thendried at about 180° C. to form a first masking layer (polyimide film).

Electrochemical Forming Step

The aluminum foil fixed to a metal-made guide was disengaged from thecoating apparatus. Thereafter, the area from the distal end to the firstmasking line of the aluminum foil was dipped in an aqueous ammoniumadipate solution and a voltage of 13 V was applied to electrochemicallyform the unformed area on the cut end portion. Thus, a dielectric filmwas formed.

Second Masking Step

The aluminum foil fixed to a metal-made guide was again mounted on thecoating apparatus and a 0.8 mm-width line was drawn by the polyimideresin solution (RIKACOAT™, trademark, produced by Shin Nippon Rika K.K.) on the portion 4 mm inside from the unfixed distal end in the samemanner as above. The solution was dried at about 180° C. to form asecond masking layer (polyimide film).

Solid Electrolyte Forming Step

In the electrochemically formed layer region exclusive of the spacebetween the first masking layer and the second masking layer, a solidelectrolyte was formed as follows.

More specifically, the area (3 mm×4 mm) opposite to the first maskinglayer side with respect to the second masking layer formed 4 mm insidefrom the distal end of aluminum foil was dipped in an isopropanolsolution containing 20 wt % of 3,4-ethylenedioxythiophene (Solution 1),pulled up and left standing at 25° C. for 5 minutes. Thereafter, thealuminum foil area treated with the monomer solution was dipped in anaqueous solution containing 30 wt % of aqueous ammonium persulfatesolution (Solution 2) and then dried at 60° C. for 10 minutes to allowoxidative polymerization to proceed. The operation from dipping inSolution 1 to oxidative polymerization by dipping in Solution 2 wasrepeated 25 times to form a solid electrolyte layer.

Cutting Step

The thus-prepared aluminum foil element having formed thereon a solidelectrolyte layer was subjected to coating of carbon paste and silverpaste in the area where the electrically conducting polymer layer wasformed, thereafter, the aluminum foil was cut off between the firstmasking layer and the second masking layer.

Fabrication and Test of Chip-Type Solid Electrolytic Capacitor Device

Three sheets of cut foil in the moiety having the second masking layerwere superposed by joining one on another with silver paste, an anodelead terminal was welded to the portion free of the electricallyconducting polymer, the whole was sealed with epoxy resin, and thedevice obtained was aged at 120° C. for 2 hours while applying a ratedvoltage. In this manner, 30 units in total of chip-type solidelectrolytic capacitors were manufactured. FIG. 7 shows a cross sectionof the thus-manufactured chip-type solid electrolytic capacitor.

These capacitor devices were subjected to a reflow soldering test bypassing each device through a temperature zone at 230° C. for 30seconds. The leakage current 1 minute after the application of a ratedvoltage was measured and an average leakage current of devices having ameasured value of 1 CV or less were determined. When the leakage currentwas 0.04 CV or more, the device was determined as defective. The resultsobtained are shown in Table 2.

EXAMPLE 7

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 6 except for using a polyimide resin solution(UPICOAT™ FS-100L, produced by Ube Industries, Ltd.) as the firstmasking material. The measurement of leakage current and the reflowsoldering test were also performed in the same manner. The resultsobtained are shown in Table 2.

EXAMPLE 8

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 6 except for using a polyimide resin solution(UPICOAT™ FS-100L, produced by Ube Industries, Ltd.) as the first andsecond masking materials. The measurement of leakage current and thereflow soldering test were also performed in the same manner. Theresults obtained are shown in Table 2.

EXAMPLE 9

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 8 except that the oxidative polymerization in thestep of forming a solid electrolyte was performed by dipping thealuminum foil in an aqueous solution prepared by further adding sodium2-anthraquinonesulfonate (produced by Tokyo Chemical Industry Co.) toSolution 2 to have a concentration of 0.07 wt %. The measurement ofleakage current and the reflow soldering test were also performed in thesame manner. The results obtained are shown in Table 2.

EXAMPLE 10

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 8 except that the oxidative polymerization in thestep of forming a solid electrolyte was performed by dipping thealuminum foil in an aqueous solution prepared by further adding sodium2-naphthalenesulfonate (produced by Tokyo Chemical Industry Co.) toSolution 2 to have a concentration of 0.06 wt %. The measurement ofleakage current and the reflow soldering test were also performed in thesame manner. The results obtained are shown in Table 2.

COMPARATIVE EXAMPLE 3

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Example 6 except that masking was performed once. Morespecifically, the chip-type solid electrolytic capacitor wasmanufactured by forming a polymer (polyimide) film only through thesecond masking step (masking at the site of 4 mm inside from the distalend of the aluminum foil) and then subjecting the aluminum foil toelectrochemical forming, formation of solid electrolyte and cutting off.The measurement of leakage current and the reflow soldering test wereperformed in the same manner. The results obtained are shown in Table 2.

COMPARATIVE EXAMPLE 4

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Comparative Example 3 except that a tape comprising a heatresistant substrate and a heat resistant adhesive was bonded in thewidth of 1 mm to the front and back surfaces of the aluminum foil inplace of forming the second masking layer. The measurement of leakagecurrent and the reflow soldering test were performed in the same manner.The results obtained are shown in Table 2.

COMPARATIVE EXAMPLE 5

Chip-type solid electrolytic capacitors were manufactured in the samemanner as in Comparative Example 3 except that a 0.8 mm-width line wasdrawn on the front and back surfaces of the foil by coating and curingphenol resin in place of forming a polymer insulating film. Themeasurement of leakage current and the reflow soldering test wereperformed in the same manner. The results obtained are shown in Table 2.

TABLE 2 Heat Resistance Failure Average Ratio in Reflow Leakage CurrentSoldering Test Example 6 0.20 μA 0/30 Example 7 0.19 μA 0/30 Example 80.17 μA 0/30 Example 9 0.16 μA 0/30 Example 10 0.16 μA 0/30 ComparativeExample 3 0.32 μA 1/30 Comparative Example 4  2.0 μA 4/30 ComparativeExample 5  2.2 μA 5/30

INDUSTRIAL APPLICABILITY

The method for producing a solid electrolytic capacitor of the presentinvention is advantageous in the following points as compared withconventional techniques.

(a) By use of polyimide resins as a masking material in place ofconventionally used tape or epoxy or phenol-type resins, the surface ofthe dielectric film is covered sufficiently and further the maskingagent infiltrates into the dielectric film to the core metal, resultingin that the electrically conducting polymer-impregnated part and theanode part can be completely separated, the leakage current can be inturn reduced, and the stress generated at the reflow treatment or thelike during or after the formation of a capacitor device can be relaxed.(b) Since the cut end part of the formed foil in the moiety iselectrochemically formed in a perfect manner, the increase of leakagecurrent due to invasion of electrically conducting polymer orelectrically conducting paste into the cut end part can be prevented.(c) By virtue of the masking layer, the electrochemically formingsolution is prevented from running out over the masking at thesubsequent electrochemically forming step and a necessary area iselectrochemically formed with ease and no fail. Further, since the solidelectrolytic capacitor of the invention has the structure such that themasking agent infiltrates into the above dielectric film and the maskingmaterial is formed on the infiltrated portion, thereby preventing thesolid electrolyte from infiltrating into the dielectric film in whichthe masking agent has already infiltrated, the cathode portion and theanode portion can be reliably insulated from each other.(d) The polyimide film used as a masking material has resistance againstwater-based solvents or organic solvents such as alcohols, used at thepolymerization of the electrically conducting polymer, therefore, theinsulation between the anode part and the cathode part can be maintainedwithout fail.

Further, according to the method that performs masking twice, the firstmasking layer (temporary masking layer) prevents the electrochemicallyforming solution from running out over the masking in the subsequentelectrochemically forming step so that a necessary area (area exclusiveof the anode part of the solid electrolytic capacitor) iselectrochemically formed with ease and without fail. More specifically,if the temporary masking is not present, the electrochemically formingsolution may run out over the substrate and the leakage current at theelectrochemical forming becomes large to cause short circuit to theelectrode. The occurrence of short circuit may be reduced by taking aspace between the position of electrochemically forming solution and theposition of electrode using a long substrate (metal foil), however,decrease in the profitability and the productivity results. In the caseof not providing a temporary masking, the amount of theelectrochemically forming solution run out is difficult to control andthe electrochemically formed state cannot be controlled. According tothe two-stage masking method of the present invention, these problemscan be solved.

Furthermore, in the method and the apparatus according to the presentinvention for coating a masking agent to a solid electrolytic capacitorsubstrate, the masking material (for example, a polyimide resin) whichis dissolved or dispersed uniformly in a solvent can be applied to thesubstrate continuously in the form of a straight line having a stablewidth each time the disk makes one rotation.

1. A method for coating a masking agent, comprising the steps of fixedlyattaching a plurality of solid electrolytic capacitor substrates to ametallic guide in a cantilever fashion, and bringing a rotating diskinto contact with the substrates at a desired position at apredetermined pressing force while the metallic guide is moved, therebycoating a masking material solution, which is fed from masking materialsolution supply means to the coating surface of the rotating disk, toopposite surfaces and opposite side surfaces of the solid electrolyticcapacitor substrate at a desired position.
 2. The method for coating amasking agent as claimed in claim 1, wherein the relative positionbetween the metallic guide and the rotating disk is inverted to therebyapply the masking material solution to opposite surfaces and oppositeside surfaces of the substrate fixedly attached to the metallic guide.